My instincts tell me it was coined in the 90's amid the disgusting/edgy phase after the long "dad jokes" phase of the years before. (Unix, byte, nybble ...the list continues)
it's quite a common term outside of software development too.
Edit: googling the term, it seems to have been coined in as early as the 1930s: https://grammarist.com/idiom/spitballing/
